1. Field of the Invention
The present invention relates to a process to clean dirty broken waste glass that is comingled with undifferentiated trash. The invention also relates to further processing of the clean glass into ground glass products.
2. State of the Art
Material recovery facilities (MRFs) receive recyclable materials. In a relatively automated process, a MRF separates recyclables collected primarily from residential curbsides into homogeneous product streams (e.g., cardboard, mixed paper, aluminum cans, etc.). In addition, such recyclable glass-containing materials can be received from other sources. Other sources include, but are not limited to, restaurant and bar bottle collection companies, construction and demolition MRFs, municipal solid waste (MSW) incinerator bottom ash, any type of glass product manufacturing process. For purposes herein, MRF shall be used to inclusively represent all of these sources.
There are two types of MRFs: single stream and dual stream. Initially, all residential MRFs were dual stream. In a dual stream MRF, paper products are collected separately from cans, bottles and plastics. A dual stream MRF has picking lines where unbroken clear and amber bottles are hand-picked and color separated, with the broken and green glass disposed. With the advancement of MRF processing technology allowing for more capital intensive automation that is cost justified by the reduction in labor effort, most dual stream MRFs converted to single stream.
For single stream MRF processing, all recyclables are comingled at the curbside. Glass is a problem in a single stream MRF because of the damage glass can cause to the processing equipment. Therefore, most single stream MRFs crush the comingled materials and screen the glass out early in the sorting and separation process, often after cardboard removal. Along with the glass comes bits and pieces of plastic, paper, ferrous and non-ferrous metals, ceramics, stones, dirt and organics such as pizza crust, etc.
Typical single stream MRF residue is 70%-85% glass, 1%-10% moisture and the remaining being undifferentiated trash. The broken glass size is typically 2 to 3 inches and smaller (2 or 3 inch minus). The plastic fraction may contain large pieces (e.g., whole plastic bottles, plastic bags) down to bottle cap size pieces and smaller. The non-ferrous material is mostly aluminum with small quantities of solid non-ferrous objects such as brass fittings. Although the moisture content is relatively low, the material appears wet because most of the moisture is contained in the paper component. Due to the paper wetness, the paper adheres to the glass and other solid components.
The current basic method to separating the glass from the non-glass components includes pulverizing the material as it is received from the MRF. This reduces the glass size from two inch minus to ⅜ inch minus without reducing the size of most of the non-glass components. A simple screen then achieves a reasonably good separation. There are, however, small bits and pieces of foreign materials as well as the liquid organic component from the glass bottle contents left behind with the glass. The fundamental problem with this method results from the size reduction. Not only does it generate a significant amount of fines (e.g., glass powder less than 40 mesh), it spreads the liquid organic fraction over a significantly larger surface area. Both of these issues cause subsequent cleaning challenges.
The cleaning has been accomplished by one of two methods: washing and baking. Washing the glass is a proven effective method, but all of the fines end up in the waste water sludge which not only loses potential glass product, the sludge is difficult to dewater and handle. The other method is baking the contaminants off in a fluidized bed dryer at 400° F. The action of fluidization causes particle-on-particle collisions, which knocks most of the baked organic material off of the glass. There must be sufficient residence time and air flow to achieve sufficient cleanliness. The problem is that the airflow required to remove the contaminants to a dust collector from the fluidized glass also removes the glass fines.
The only known installation using fluidized heat to clean dirty MRF glass so far has been unable to produce sufficiently clean glass on a consistent basis. A sample of purportedly clean residue was obtained from the installation and subjected to testing under a loss on ignition (LOI) test. Such a test, measures how much of a product is lost, by weight, upon burning. Where there is a high organic component, the loss will be relatively high. When several samples were tested, the overall loss was measured at 1.44%. While this may not seem high, such a result indicates that the purportedly clean glass still contained unsuitably high organics for various end-uses, including use as a pozzolan.